Interpretive Summary: Computer simulation models can allow efficient comparisons between natural grasslands and improved pastures. Such models need components for plant growth, soil water balance, runoff, soil erosion, and climatic impacts. In this study, we developed plant parameters that enable the ALMANAC model to simulate growth of coastal bermudagrass, bahiagrass, and some common native, warm season grasses. These parameters described leaf area, light interception, biomass growth, and nitrogen concentration for these grasses, derived from measurements at three field sites in Texas for multiple years. The maximum leaf area index of coastal bermudagrass and bahiagrass were near 2.2. Those for native grasses other than switchgrass were much less. Values describing efficiency of light interception were greatest for little bluestem and blue grama and least for switchgrass. The amounts of plant dry matter produced per unit light intercepted were lower than commonly reported for crops. Grass nitrogen concentration values showed similar patterns of change among species during the seasons. These parameters hold promise to enable realistic simulation of improved and native grasses by the ALMANAC model.

Technical Abstract:
Effective comparisons of natural grasslands and improved pasture requires a robust model for plant growth, soil water balance, runoff, soil erosion, and climatic impacts. The objective of this study was to develop plant parameters that enable the ALMANAC model to simulate growth of coastal bermudagrass (Cynodon dactylon (L.) Pers.), bahiagrass (Paspalum notatum Flügge var saurae Parodi) and some common native, warm season grasses. We derived parameters describing leaf area, light interception, biomass growth, and nitrogen concentration for these grasses by taking measurements at three field sites in Texas for multiple years. The maximum LAI values (LAImx) of coastal bermudagrass and bahiagrass were near 2.2. Those for native grasses other than switchgrass were much less. Mean values for light extinction coefficient (k) were 1.0 to 1.1 for coastal bermudagrass, bahiagrass, and sideoats grama (Bouteloua curtipendula (Michaux) Torrey), while the switchgrass (Panicum virgatum L.) value was near 0.7. The k values of the other species were 1.3 for little bluestem (Schizachyrium, scoparium (Michaux) Nash), 1.6 for blue grama (Bouteloua gracilis (H.B.K.) Laga. ex Steud.), and 2.1 for buffalograss (Buchloe dactyloides (Nutt.) Engelm.). Radiation use efficiency (RUE) values for four of the five measured grass species were between 1.0 and 2.0 g MJ-1. Grass [N] values showed similar patterns of change among species during the seasons, with three of the five measured species decreasing nonlinearly to near 0.010 g per g dry matter by the end of the growing season. These parameters hold promise to enable realistic simulation of improved and native grasses by the ALMANAC model.